1. Field of the Invention
The present invention relates to calibration of communication channel parameters in systems, including mesochronous systems, in which two (or more) components communicate via an interconnection link; and to calibration to account for drift of conditions related to such parameters during operation of the communication channels.
2. Description of Related Art
In high-speed communication channels, which are operated in a mesochronous manner, typically a reference clock provides frequency and phase information to the two or more components on the link. A transmitter on one component and a receiver on another component each connect to the link. The transmitter and receiver operate in different clock domains, which have an arbitrary (but fixed) phase relationship to the reference clock. The phase relationship between transmitter and receiver is chosen so that the propagation delay seen by a signal wavefront passing from the transmitter to the receiver will not contribute to the timing budget when the signaling rate is determined. Instead, the signaling rate will be determined primarily by the drive window of the transmitter and the sample window of the receiver. The signaling rate will also be affected by a variety of second order effects. This system is clocked in a mesochronous fashion, with the components locked to specific phases relative to the reference clock, and with the drive-timing-point and sample-timing-point of each link fixed to the phase values that maximize the signaling rate.
These fixed phase values may be determined in a number of ways. A sideband link may accompany a communication link (or links), permitting phase information to be passed between transmitter and receiver. Alternatively, an initialization process may be invoked when the system is first given power, and the proper phase values determined by passing calibration information (patterns) across the actual link. Once the drive-timing-point and sample-timing-point of each link has been fixed, the system is permitted to start normal operations.
However, during normal operation, system and environmental conditions will change. Ambient temperature, humidity, component temperature, supply voltages, and reference voltages will drift from their initial values. Many of the circuits in the components will be designed to be insensitive to drift within a specified range, but the drift will need to be considered when setting the upper signaling rate of a link.
As the conditions drift, the optimal timing points of the transmitter and receiver will change. If the timing points remain at their original values, then margin must be added to the timing windows to ensure reliable operation. This margin will reduce the signaling rate of the link.
To improve performance, prior art systems perform a periodic calibration process. The periodic calibration process consumes resources of the communication channel, and therefore reduces the performance of the communication channel for its mission functions.
In many systems, the communication channel is considered asymmetrical, because one side has greater resources for supporting the calibration process than the other. For example, in a memory system, including a memory controller and one or more memory devices such as high-speed DRAMs, the memory controller may include the drivers that are adjustable in response to calibration resources, while the memory devices have lower-cost drivers that cannot be adjusted. In such a system, the memory controller must execute a calibration process considered open-ended, taking into account the asymmetrical nature of the communication channel, without active adjustments of signaling characteristics by the targets of the communication. Such calibration processes can consume greater channel resources than might be required for closed channels. Also, open-ended channels do not gracefully track shifts in clock frequencies of spread spectrum clocks nor do they function well in plesiochronous systems where the frequencies of the respective clocks on the two components are offset by a small delta.
It is desirable to provide techniques for calibration of communication channels which provide more efficient utilization of system resources, without requiring costly circuitry on both sides of the communication channels.